Also electric leakage detecting devices for shutting off AC electric circuits may detect respective peak values of positive and negative output signals of a zero-phase current transformer that are proportional to a ground-fault current, determine positive and negative signal widths from positive and negative peak value signals, integrate the positive and negative signal width signals, and output a trip control signal based on integration results of the positive and negative signals to operate a ground-fault interrupter (refer to JP-1991-226225-A, for example).
Electric leakage detecting devices may be equipped with a computing circuit which converts an output signal of a zero-phase current transformer that is proportional to a ground-fault current into a pulse signal by an integration circuit and a VF (voltage-frequency) conversion circuit and generates a trip control signal for shutting off an AC electric circuit if an integration value of the pulse signal is larger than or equal to a prescribed value (refer to JP-2004-220859-A, for example).
According to the above-kinds of electric leakage detecting devices, an undesirable operation is caused due to erroneous detection when harmonic noise or a surge is superimposed on an output signal of the zero-phase current transformer. Furthermore, detection of a signal level that is proportional to a ground-fault current requires an analog comparator and cannot be given high detection accuracy because the detection accuracy depends on the accuracy of the analog comparator. Still further, no indication is provided as to how to detect a leakage current having a load-rectified waveform.